The present disclosure relates to fabrication of semiconductor devices having accurately formed designed features, more particularly to monitoring focus for die patterns formed during the fabrication process.
The formation of various integrated circuit (IC) structures on a wafer often relies on lithographic processes, sometimes referred to as photolithography, or simply lithography. As is well known, lithographic processes can be used to transfer a pattern of a photomask (also referred to herein as a mask or a reticle) to a wafer. Patterns can be formed from a photoresist layer disposed on the wafer by passing light energy through a reticle mask to form an image of the desired pattern onto the photoresist layer. As a result, the pattern is transferred to the photoresist layer. Such pattern formations are performed by devices well known as optical steppers or scanners. Conventional photomasks, commonly referred to as chrome on glass (COG), consist of chromium patterns on a quartz plate. Light of a specific wavelength is projected through the spaces between the chromium patterns onto the wafer. As the chromium portions block light transmission, such photomasks are known as binary masks.
In areas where a positive type photoresist is sufficiently exposed, after a development cycle the photoresist material can become soluble such that it can be removed to selectively expose an underlying layer (e.g., a semiconductor layer, a metal or metal containing layer, a dielectric layer, a hard mask layer, etc.). Portions of the photoresist layer not exposed to a threshold amount of light energy will not be removed and will serve to protect the underlying layer during further processing of the wafer (e.g., etching exposed portions of the underlying layer, implanting ions into the wafer, etc.). If a negative type photoresist is used, unexposed portions are selectively removed. After the wafer fabrication process for this pattern is performed, the remaining portions of the photoresist layer can be removed from the underlying substrate.
There is a continuing objective to increase the density with which various integrated circuit structures are arranged. To this end, feature size, line width, and the separation between features and lines are being made smaller. Fabrication in the sub-micron range incurs limitations on the faithful reproduction of reticle patterns as exposed images on the photoresist layer. Yield is affected by factors such as mask pattern fidelity, optical proximity effects, photoresist processing and tool precision. These concerns are largely dependent on local pattern density and topology. In describing the lithographic yield, exposure latitude and depth of focus are two critical elements that describe allowable margin of exposure light dose and defocus amount that can print the circuitry without generating a failure in functionality of the chip. As the required pitch continues to be reduced, the depth of focus of the projected light has significantly decreased.
In current photo-lithography technology, application of off-axis illumination of shorter wavelength has become necessary to provide appropriate process margin for minimum pitch. In order to obtain maximum process window, it is thus imperative to determine the best focus plane so that the position of the photoresist can be adjusted to a critical dimension to place the focus within the process margin. One known approach is by exposing a matrix field through a range of focus settings, then inspecting the resultant pattern for the best-focused images. This procedure is time consuming and subject to error.
Another approach employs a phase shift mask that includes a transparent substrate, a light-shielding film having a predetermined pattern, and a phase shifter formed on the predetermined pattern. As a result of the phase shifter, an image pattern is formed on the photoresist that shifts along its surface plane as the substrate wafer is moved in the direction of the illumination axis. Measurement of the amount of shift of the pattern image in a direction along the surface enables correlation with a position in the axis direction, thereby providing a measurement of focus. The phase shift mask is a complex structure which presents greater difficulty in forming patterns on both sides of the mask than formation of a binary mask.
Other phase shift mask focus monitoring methods employ a plurality of phase shift masks, each requiring a separate exposure at a special aperture stop. The photoresist additionally must be exposed at the aperture used for chip fabrication.
Phase shift monitoring thus adds a significant manufacturing cost as well as lengthening the entire process. A need thus exists for a monitoring method and system that can provide accurate focus detection, more simply and efficiently, and at less manufacturing cost, than systems currently available.